The present invention generally relates to step motors, and, more particularly, to a step motor which has a position detector for correctly detecting a position of one of a plurality of pole teeth provided on a rotor of the motor and a method for recording a signal indicative of a position of a rotor in the motor.
A step motor is suitably used when it is desired to make a motor more compact, because the step motor exhibits high torque characteristics at low speeds and can directly drive a load without intervention of a reduction mechanism. As one example of a simple step motor there is known an open loop control type motor which is relatively easily controlled. This open loop control type motor is also referred to as a step motor, and is arranged so that a pulse current having a predetermined frequency is supplied to a stator winding to rotate a rotor in synchronism with the pulse current. In order to accurately follow the rotation of a motor in accordance with a commanded speed and to avoid any step-out, there has been proposed a step motor of a closed loop control type in which the rotational speed of a rotor or the position of one of a plurality of pole teeth provided on the rotor is detected.
In this closed loop control type motor, it is necessary to commutate a current flowing through a phase winding of the stator in association with the position of the pole tooth of the rotor and a position detector for detecting the position of the pole tooth is also required.
FIG. 1 shows an ordinary step motor provided with such a position detector. In the drawing, the step motor includes a cylindrical housing 1 made of steel plate and a stator 2 of laminated silicon steel sheets disposed inside the housing 1. The stator 2, as shown in FIG. 2, is provided with 16 inwardly-projected poles 2A each having 5 pole teeth 2B at their tip ends. Provided inside the stator 2 is a rotor 5 which is rotatably journalled in end brackets 3 and 4 disposed at both ends of the housing 1. More specifically, the rotor 5 is rotatably journalled in bears 6 and 7 provided in the end brackets 3 and 4.
The rotor 5 comprises an exciting magnet 8 and rotor yokes 9 and 10 provided at both sides of the exciting magnet 8 so as to hold the magnet 8. The rotor yokes 9 and 10 are provided at their outer peripheries with pole teeth 5B which are opposed to the pole teeth 2B provided at the inner side of the stator 2 as shown in FIG. 3. More specifically, the rotor yokes 9 and 10 are fixedly mounted on a shaft 11 in such a relative rotational position relationship as to be mutually shifted by an amount corresponding to 1/2 of the pitch of the pole teeth. The pitch of the pole teeth formed in the stator is set to be equal to that formed in the rotor. The projected poles 2A are actually wound with a phase winding therearound but the phrase winding is omitted in FIG. 2. For the winding, the two-phase, three-phase or multi-phase winding can be selected to control the energization of the projected poles, depending on the specifications of the motor to be employed. The exciting magnet 8 is magnetized as shown by Ns and Ss along the axial direction in FIG. 1. The shaft 11 is outwardly extended beyond the bearings and in the illustrated example, for example, a left-side of the shaft 11 is used as an output shaft which is drivingly connected to a load while a right-side thereof is used for detection. Attached onto the right-side end of the shaft 11 is a position detecting plate 12 which detects a position of the pole teeth 5B on the outer periphery of the yoke 9 or 10 of the rotor 2 and which generates a commutation signal for a phase current flowing through the winding wound around the projected poles 2A. The position detecting late 12 has exactly the same sectional shape as the rotor yoke 9 or 10 when viewed from the axial direction. That is, the position detecting plate 12 is provided at its outer periphery with teeth which have the same number, shape and pitch as the pole teeth 5B provided at the outer peripheries of the rotor yokes 9 and 10. More specifically, since the position detecting plate 12 is set to have the same positional relationship, the position of the rotor yoke 9 or 10, i.e., the position of the stator to the projected poles, can be detected by detecting the position of the position detecting plate 12. As the method for detecting the teeth of the position detecting plate 12, there are known optical and magnetic methods for optically and magnetically detecting the teeth of the position detecting plate 12. In the event where the position detecting plate 12 is made of a non-magnetic material such as aluminum or plastic, light emitting and receiving elements are disposed at both sides of the outer peripheral teeth of the position detecting plate 12 so that the light receiving element detects the passage and interruption of light caused by the rotation of the position detecting plate 12 and converts it into an electrical signal to thereby detect the teeth of the position detecting plate 12. In the case where the position detecting plate 12 is made of a magnetic material, an element for magnetically detecting the tooth part of the position detecting plate 12, e.g., a magnetic reluctance effect element of a reactance type having a bias magnet carried on its substrate or a Hall-effect element is employed. Such an element is omitted in FIG. 1.
A speed detecting plate 14, which is made of a nonmagnetic material such as aluminum, is fixedly mounted on the right end of the shaft 11. A magnetic recording medium 15 is coated on the outer periphery of the speed detecting plate 14 to record thereon many magnetic poles (magnetic signals). A speed detector 16, which is fixed to the end bracket 3, comprises a magnetic reluctance effect element for detecting the magnetic poles magnetized on the magnetic recording medium 15. As the motor rotates, the resistive value of the magnetic reluctance effect element is varied. Thus, the speed of the motor can be detected by extracting a variation in the resistive value of the magnetic reluctance effect element, for example, in the form of an electrical signal and by measuring the number of pulses in the signal within a predetermined sampling time or a time interval corresponding to two or more pulses.
A hybrid type step motor provided with such a position detector and a speed detector as mentioned above is known.
This type of step motor has the advantages of low speed and high torque but also has the technical disadvantage of not having a smooth rotation.
In order to overcome such a demerit, in the case of a two-phase step motor, it is considered to provide sinusoidal currents in phase with sinusoidal induction voltages which are generated in two phase windings and mutually phase-shifted by an electrical angle of 90 degrees as shown in FIG. 4. In this case, a torque T generated by the motor is expressed as follows; EQU T.varies.C(sin.sup.2 .theta.+cos.sup.2 .theta.)=C
where C denotes a constant. Hence the rotation of the motor become smooth.
Explanation will next be made as to the schematic arrangement of a general control circuit for this type of step motor by referring to FIG. 5.
In the drawing, a current command circuit 52 comprises a read only memory (ROM) in which sinusoidal current commands as mentioned above are previously stored. A counter 51 counts a speed detection signal detected by the speed detector 16 coupled to a motor 50, and according to a speed determined based on the detected speed signal, a corresponding one of the sinusoidal signals previously stored in the current command circuit 52 is output to a D/A converter 53. An automatic current restricter 54 adjusts an analog output value of the D/A converter 53 and applies it to a pulse width modulation (PWM) circuit 55 of the next stage. The PWM circuit 55 applies an energization ratio (duty ratio) corresponding to the analog value commanded by the previous stage (automatic current restricter 54) to a drive circuit 56 which in turn comprises an inverter to drive motor 50 at a rotational speed commanded by the current command circuit 52.
However, driving the motor merely based on the instructed current command cannot ensure that the speed of the motor is actually maintained at the instructed speed. To this end, the aforementioned speed detector 16 is provided on the shaft 11 of the motor. That is, as the motor 50 rotates, the speed detector 16 can generate a sinusoidal signal having a frequency corresponding to the current rotational speed of the motor.
The sinusoidal signal of the speed detector 16 is converted to a pulse signal through a shaping circuit (not shown) and then sent to a microprocessor 58 where a speed corresponding to the pulse signal is calculated. The calculated speed is then sent from the microprocessor 58 to the D/A converter 53. In the D/A converter 53, the current (speed) command value received from the current command circuit 52 is compared with the calculated actual speed value received from the microprocessor 58 to generate the current command on the basis of the comparison difference. A current detecting element 59 detects a supply current to the motor and applies it to the automatic current restricter 54. The rotor position detector 13 generates an output signal corresponding to the pole tooth 5B which signal is used as a reset signal for the count value of the counter 51. That is, the respective pole teeth can cause an identical current command to be provided to the motor 50. Since the current command is digitally processed, the accuracy of generation and detection of the rotor position detection signal is only required to be within one count value of the counter 51.
In order to smoothly rotate the motor at very low speeds without any irregular rotation, however, the resolution of the speed detector 16 must be set to be high, and thus it is necessary to set the resolution (the number of pulses generated per one turn) of the speed detector 16 at at least 3600FC (fine count).
In this way, when the speed detector 16 having a high resolution is employed, this also requires the position of the pole tooth 5B of the rotor to be detected at a high accuracy without any error.
In the case of such a position detector as shown in FIG. 1, the position detecting plate 12 having the same shape as the rotor yoke 9 or 10 is used to detect the outer peripheral pole tooth 5B of the rotor 5, thus involving an accuracy problem on the rotational position alignment between the rotor 5 and position detecting plate 12. Further, it is impossible to make the position detecting plate 12 in exactly the same shape as the rotor yoke 9 or 10, which results in that the pole tooth 5B of the rotor cannot be correctly detected without any error.
For the purpose of solving such problems, there has been proposed a step motor as disclosed in JP-A-61-69364, in which a rotor position detecting projection pole is provided for directly detecting projection poles provided at the outer periphery of the rotor and a winding is wound around the rotor position detecting projection pole. In this case, the position detecting projection pole detects the projection poles formed at the outer periphery of the rotor directly through the position detecting winding and uses the detected signal for the phase switching of the winding.
Such an arrangement is considered to be effective, since the position of the rotor projection poles can be directly detected by means of the winding, and thus the correct position of the projection poles can be detected.